POST-DARWINIAN VIEWS 69 



ism in complete mutual separation, and these sepa- 

 rate characters, called "allelomorphs," are distributed 

 in the germ-cells in such wise that all possible combi- 

 nations of them are present in approximately equal 

 numbers. To restate this in other terms, the germ- 

 cells of opposite parents which are found in hybrid 

 plants each contain one or other only of any pair of 

 differentiating characters possessed by the parents; 

 and each member of such a pair of characters is con- 

 tained in an equal number of germ-cells of both sexes. 

 By a pair of characters is meant two characters that 

 can only be described by their mutual differences: — 

 for instance, smooth seeds and wrinkled seeds. It 

 should be added that the separate pairs of differentiat- 

 ing characters or allelomorphs obey Mendel's law in 

 complete and mutual independence. This mutual 

 separation, along with the numerical law of their com- 

 binations in germ-cells, determines the results of cross- 

 breeding; and the practical application of Mendel's 

 discovery enables cross-breeders to select desirable 

 variations in such a manner as to produce varieties 

 which breed true and hold their own.^ 



The facts upon which Mendel's law is based consti- 

 tute an important part of the data by which Hugo de 

 Vries was led to maintain the evolutionary theory of 

 heterogenesis, commonly called the mutations theory. 

 This theory had been enunciated by several earher 



* On Mendel's law, see R. H. Lock, op. cit., chh. vii, viii; Bateson, 

 Mendel's Prins. of Heredity (which gives translations of Mendel's 

 own papers); Baldwin's Die. of Philos., s. v. "Evolution (Mendel)." 



